Communication systems typically employ various multiple access schemes in order to accommodate multiple users sharing a limited amount of bandwidth, while maintaining a certain amount of throughput. Some common multiple access schemes include Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), etc. Satellite communication systems also use multiple access schemes to provide communication services (e.g., voice, data, etc.) to subscribers. Latency sensitive applications, however, can impose disadvantages for satellite communication systems because of the long round trip delay associated with transmission via geosynchronous satellites.
TDMA-based Satellite communication systems include a gateway which allocates bandwidth to individual terminals (i.e., satellite terminals) at a specific time and frequency on a satellite return channel (also referred to as the “inroute”). Bandwidth allocation is based on demand reported by the terminals. Due to the long round trip delay, however, the demand based bandwidth allocation method may not meet the delay requirement for latency sensitive traffic.
The TDMA based Aloha channel/RACH (Random Access Channel) can be used for control messages and latency sensitive traffic. For example, many systems transmit control messages on an Aloha/RACH channel, and transmit data (including voice) on a channel assigned based on demand or predictive use. Some systems also transmit a small high priority data on the Aloha/RACH channel. Since only one terminal can successfully transmit in the same time and frequency, collisions can occur when using the Aloha/RACH channel. Thus, bursts allocated to Aloha can only be used with an efficiency of about 10-15% with a certain acceptable collision rate, while regular TDMA achieves much higher efficiencies. Even if slotted Aloha is used, the maximum throughput is only about 36.8%, given 1:1 load density.
Scrambled Coded Multiple Access (SCMA) is an innovative technology that allows multiple users to simultaneously share a channel of the same frequency at the same time with much higher success rate. This is achieved, in part, by using a method of interference cancellation. In SCMA, for example, a physical (PHY) layer burst is heavily protected by a very low code rate (e.g., rate 1/9) and a strong Unique Word (UW) for burst identification. SCMA is discussed in greater detail in U.S. Pat. No. 8,683,292, entitled “Method and System for Providing Low Density Parity Check (LDPC) Coding For Scrambled Coded Multiple Access (SCMA),” and US Patent Application Publication No. 2014/0201602, entitled “Method and System for Providing Scrambled Coded Multiple Access (SCMA),” the contents of which are incorporated by reference.
SCMA can advantageously improve spectrum efficiency compared to Aloha TDMA, and also improve latency performance compared to regular stream TDMA. An SCMA channel could have 5.5 times or higher spectrum efficiency than a TDMA Aloha channel. Additionally, SCMA allows terminals to transmit autonomously without an explicit bandwidth allocation, thereby significantly improving the latency performance. Based on the foregoing, it would be advantageous to provide an approach for a satellite communication system which utilizes both SCMA and TDMA. SCMA can be used to transmit latency sensitive traffic, while assigned TDMA slots are assigned to terminals for high throughput traffic.